Rafter Calculation Formulas
Rafter Length = Rafter Run ÷ cos(Roof Angle)
Total Length = Rafter Length + Overhang ÷ cos(Roof Angle)
Ridge Cut Angle = 90° - Roof Angle
Tail Cut Angle = Roof Angle
Birds-mouth Length = √(Plumb² + Seat²)
Note: Allow for ridge thickness when determining rafter run to outer wall. Check local regulations for appropriate sizes.
Quick Answer: How to Calculate Rafter Length
Rafter length = √(run² + rise²) + overhang. For a roof with 4m run and 2m rise (a 26.6° pitch), the rafter is √(16+4) + overhang = 4.47m + overhang. The plumb cut angle equals the roof pitch; the seat (birdsmouth) cut is the same angle.
For Australian framing per AS 1684: rafter spacing is typically 450 or 600mm centres, sized from rafter span tables based on roof load, snow zone, and timber stress grade.
Use the calculator above to get rafter length, plumb cut, birdsmouth, and overhang in seconds.
Roof Rafter Calculator — Free Rafter Length Calculator Australia
Calculate rafter length, rise, run, and birdsmouth cut dimensions for any roof pitch. Our free rafter length calculator is designed for Australian builders working with common roof pitches from 5 to 45 degrees. Enter your span and pitch to get instant results.
How to Calculate Rafter Length
The basic rafter length calculation uses trigonometry:
Rafter Length = Run / cos(Pitch Angle)
Or equivalently:
Rafter Length = sqrt(Run² + Rise²)
Where:
- Run = half the building span (for a symmetrical roof)
- Rise = Run x tan(Pitch Angle)
- Rafter Length = the hypotenuse from wall plate to ridge
Example: For a 10m span building with a 25-degree pitch:
- Run = 5,000mm
- Rise = 5,000 x tan(25) = 5,000 x 0.4663 = 2,332mm
- Rafter Length = 5,000 / cos(25) = 5,000 / 0.9063 = 5,517mm
Add the eave overhang length to get the total rafter length including the tail.
Rafter Calculation Reference Table
| Pitch | Rise per m Run | Rafter per m Run | Common Roofing |
|---|---|---|---|
| 5 deg | 87mm | 1,004mm | Colorbond, skillion |
| 10 deg | 176mm | 1,015mm | Metal deck roofing |
| 15 deg | 268mm | 1,035mm | Minimum for concrete tiles |
| 20 deg | 364mm | 1,064mm | Standard metal roofing |
| 22.5 deg | 414mm | 1,082mm | Popular residential |
| 25 deg | 466mm | 1,103mm | Standard tile roof |
| 30 deg | 577mm | 1,155mm | Steep tile/slate |
| 35 deg | 700mm | 1,221mm | Traditional pitched |
| 45 deg | 1,000mm | 1,414mm | Very steep/decorative |
Birdsmouth Cut Details
The birdsmouth (or bird’s beak) is the notch cut into a rafter where it sits on the wall plate. It consists of two cuts:
- Seat cut (horizontal): Sits flat on the top plate. Length should be at least 2/3 of the plate width but no more than the full plate width
- Plumb cut (vertical): The vertical part of the notch. Must not reduce rafter depth by more than 1/3
Birdsmouth depth formula: Depth = Seat Cut Length x tan(Pitch Angle)
For a 90mm top plate with a 25-degree roof, the seat cut is typically 60-90mm and the plumb cut depth is about 28-42mm.
Common Australian Roof Pitches
- 5-10 degrees: Colorbond and metal deck roofing. Minimum pitch for most metal roofing profiles. Popular for modern flat-roof designs and skillion extensions
- 15-20 degrees: Minimum pitch for concrete and terracotta tiles. Common on project homes across Australia
- 22.5-25 degrees: The most popular pitch range for Australian residential roofs. Good balance of aesthetics, rain runoff, and usable roof space
- 30-35 degrees: Traditional steep pitch. Used in heritage areas and for slate or shingle roofing
- Above 35 degrees: Uncommon in Australia except for decorative features, dormers, and turrets
Rafter Timber Sizing Guide (AS 1684)
| Rafter Size | Max Span (N1/N2) | Max Span (N3) | Common Use |
|---|---|---|---|
| 90×35 F7 | 2.0m | 1.7m | Verandah rafters |
| 120×35 F7 | 2.8m | 2.4m | Small roof spans |
| 140×35 F7 | 3.3m | 2.8m | Standard residential |
| 190×35 F7 | 4.4m | 3.8m | Large spans |
| 240×45 F7 | 5.8m | 5.0m | Long spans, heavy loads |
Spans are approximate for sheet metal roofing at 600mm centres. Tile roofs and wider spacing require larger sizes. Always verify against AS 1684 span tables for your specific situation.
Rafter Spacing and Load Considerations
- Standard spacing: 600mm centres for most residential roofs
- Heavy loads: 450mm centres for concrete tile roofs or in cyclonic areas (C1-C4)
- Light loads: 900mm centres may be acceptable for metal roofing in low-wind areas with appropriately sized rafters
- Eave overhang: Standard 450-600mm. Maximum depends on rafter size and wind classification
“Always measure your rafter length from the outside edge of the wall plate to the centre of the ridge, then add the eave overhang separately. Common mistake is to measure from the inside of the wall, which gives you a rafter that is too short.”
— Roof carpenter, 25+ years experience, QLD
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